This invention relates generally to methods of forming quantum dots. More particularly, this invention relates to methods of forming quantum dots of Group IV semiconductor materials.
Over the past several years, there has been an increasing interest in exploiting the extraordinary properties associated with quantum dots. As a result of quantum confinement effects, properties of quantum dots can differ from corresponding bulk values. These quantum confinement effects arise from confinement of electrons and holes along three dimensions. For instance, quantum confinement effects can lead to an increase in energy gap as the size of the quantum dots is decreased. Consequently, as the size of the quantum dots is decreased, light emitted by the quantum dots is shifted towards higher energies or shorter wavelengths. By controlling the size of the quantum dots as well as the material forming the quantum dots, properties of the quantum dots can be tuned for a specific application.
Previous attempts at forming quantum dots have largely focused on quantum dots of direct band gap semiconductor materials, such as Group II-VI semiconductor materials. In contrast to such direct band gap semiconductor materials, Group IV semiconductor materials such as Si and Ge have energy gaps, chemical properties, and other properties that render them more desirable for a variety of applications. However, previous attempts at forming quantum dots of Si or Ge have generally suffered from a number of shortcomings. In particular, formation of quantum dots of Si or Ge sometimes involved extreme conditions of temperature and pressure while suffering from low yields and lack of reproducibility. And, quantum dots that were produced were generally incapable of exhibiting adequate levels of photoluminescence that can be tuned over a broad spectral range. Also, previous attempts have generally been unsuccessful in producing quantum dots of Si or Ge that are sufficiently stable under ambient conditions or that can be made sufficiently soluble in a variety of matrix materials.
It is against this background that a need arose to develop the quantum dots and methods for forming quantum dots described herein.
In one innovative aspect, the present invention relates to a method of forming a quantum dot. In one embodiment, the method comprises providing a particle that includes a semiconductor material Y selected from the group consisting of Si and Ge. The method also comprises applying sound energy and light energy to the particle to form a quantum dot that exhibits photoluminescence with a quantum efficiency that is greater than 10 percent. The quantum dot includes a core, and the core includes Y.
In another embodiment, the method comprises reacting, in a reaction medium, a source of a semiconductor material Y selected from the group consisting of Si and Ge with a reducing agent to form a particle that includes a core. The core includes Y, and the reducing agent is selected from the group consisting of Group IIA metals, transition metals, and lanthanides. The method also comprises reacting the particle with a source of surface ligands to form a ligand layer surrounding the core to form a quantum dot. The ligand layer includes at least one surface ligand.
In another innovative aspect, the present invention relates to a method of forming quantum dots. In one embodiment, the method comprises providing particles having a first peak size. The particles include a semiconductor material Y selected from the group consisting of Si and Ge. The method also comprises applying light energy to the particles for a time sufficient to form quantum dots having a second peak size. The second peak size is smaller than the first peak size.